So far, when as a superconducting tunnel junction, i.e., Josephson junction, a SIS type Josephson junction in which an oxide film (I) is sandwiched between a pair of superconductor layers (S) is to be made, it is necessary to perform three times film forming steps that a superconductor layer, an insulating barrier layer and a superconductor layer are deposited in this order (see, e.g., Nonpatent Reference 1). The property of superconducting tunnel junction is characterized by the so-called IcRN product. Larger this value, the better a junction ensued.
The single flux quantum device (hereinafter, referred to conveniently as “SFQ device”) is a logic circuit using the Josephson junction (see Nonpatent Reference 2). In a SFQ device, the logic circuit is configured utilizing a pulsed voltage generated when the Josephson junction is traversed by a single flux quantum. The switching speed in this case is inversely proportional to the IcRN product. Thus, the larger the IcRN product, the higher the speed at which it operates. The IcRN product increases generally in proportion to the critical temperature (Tc) of a superconductor used in the Josephson junction.
Nonpatent Reference 1: Gomez Espinoza Luis Beltran, doctoral thesis, USA, Cincinnati University, submitted May 30, 2003;
Nonpatent Reference 2: S. Yorozu “Single Flux Quantum Device”, Solid-State Physics, Vol. 40, No. 10, p. 807, 2005